1. Field
This invention relates generally to steam generators and more particularly to a flow restrictor for a steam outlet nozzle of a steam generator.
2. Description of Related Art
A pressurized water nuclear reactor steam generator typically comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a divider plate that cooperates with the tube sheet and a channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with a primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header and through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., the side of the generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators. This mixture called the downcomer flow, is conducted down the annular chamber adjacent the shell until the tube sheet located at the bottom of the annular chamber causes the water to change direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam, and the steam vapor then exits the steam generator through a steam exit nozzle and is typically circulated through a turbine to generate electricity in a manner well known in the art.
Nuclear steam generators typically have flow limiters in the steam exit nozzles to limit the amount of steam that will be exhausted into the containment in the unlikely event of a steam line break. While the flow limiters are an important safety feature they add a penalty that can detract from the efficiency of the generators. The full power pressure drop in the steam nozzle flow limiter of some steam generators can be nearly 20 psi. A pressure drop reduction to 5 psi or less in the steam nozzle flow limiter can have a significant benefit to the outlet steam pressure, and thus favorably increase plant efficiency and revenues. In addition, a reduction in the pressure drop reduces moisture carryover by up to 0.05 percent, which benefits turbine longevity.
Accordingly, a new flow limiter design is desired that will have a significantly lower pressure drop without sacrificing performance.
Further, such a new steam nozzle flow limiter design is desired that will have significantly lower moisture carryover.